Semiconductor devices and other microelectronic devices are typically manufactured on a workpiece having a large number of individual dies (e.g., chips). Each wafer undergoes several different procedures to construct the switches, capacitors, conductive interconnects and other components of a device. For example, a workpiece can be processed using lithography, implanting, etching, deposition, planarization, annealing, and other procedures that are repeated to construct a high density of features. One aspect of manufacturing microelectronic devices is evaluating the workpieces to ensure that the microstructures are within the desired specifications.
Scatterometry is one technique for evaluating several parameters of microstructures. With respect to semiconductor devices, scatterometry is used to evaluate film thickness, line spacing, trench depth, trench width, and other aspects of microstructures. Many semiconductor wafers, for example, include gratings in the scribe lanes between the individual dies to provide a periodic structure that can be evaluated using existing scatterometry equipment. One existing scatterometry process includes illuminating such periodic structures on a workpiece and obtaining a representation of the scattered radiation returning from the periodic structure. The representation of return radiation is then analyzed to estimate one or more parameters of the microstructure. Several different scatterometers and methods have been developed for evaluating different aspects of microstructures and/or films on different types of substrates.
Eldim Corporation of France manufactures devices that measure the photometric and calorimetric characteristics of substrates used in flat panel displays and other products. The Eldim devices use an Optical Fourier Transform (OFT) instrument having an illumination source, a beam splitter aligned with the illumination source, and a first lens between the beam splitter and the sample. The first lens focuses the light from the beam splitter to a spot size on the wafer throughout a large range of angles of incidence (e.g., Φ=0° to 360° and Θ=0° to 88°). The light reflects from the sample, and the first lens also focuses the reflected light in another plane. The system further includes an optical relay system to receive the reflected light and a sensor array to image the reflected light. International Publication No. WO 2005/026707 and U.S. Pat. Nos. 6,804,001; 6,556,284; 5,880,845; and 5,703,686 disclose various generations of scatterometers. The scatterometers set forth in these patents are useful for assessing the photometric and colorimetric properties of flat panel displays, but they may have several drawbacks for assessing parameters of extremely small microstructures on microelectronic workpieces.
One challenge of scatterometry is properly locating very small microstructures on a workpiece. This is not particularly difficult for analyzing the pixels of a flat panel display because measuring the photometric and calorimetric properties of such substrates merely requires locating the illumination spot on relatively large pixel areas instead of very small periodic structures. As a result, systems used to analyze flat panel displays may not include navigation systems capable of locating very small microstructures on the order of 20-40 μm. Moreover, the devices used to analyze flat panel displays may have relatively large spot sizes that are not useful to measure the properties of a 20-40 μm grating because such large spot sizes generate reflections from the surrounding areas that result in excessive noise. Therefore, devices designed for assessing flat panel displays may not be well-suited for assessing gratings or other microstructures having much smaller dimensions on microelectronic workpieces.
Another challenge of using scatterometry to evaluate very small microstructures is obtaining a useful representation of the radiation returning from such microstructures. Existing scatterometers that assess the films and surface conditions of flat panel displays typically use relatively long wavelengths of light (e.g., 532 nm). In contrast to flat panel displays, many microstructures on semiconductor wafers have line widths smaller than 70 nm, and such microstructures are continually getting smaller and being packed in higher densities. As a result, the relatively long wavelengths used to assess flat panel displays may not be capable of assessing very small microstructures on many microelectronic devices. Therefore, devices used for assessing flat panel displays may be further inadequate for assessing the properties of microstructures on microelectronic workpieces.
Another challenge of assessing microstructures using scatterometry is processing the data in the representation of the return radiation. Many scatterometers calculate simulated or modeled representations of the return radiation and then use an optimization regression to optimize the fit between the simulated representations and an actual reflectance signal. Such optimization regressions require a significant amount of processing time using high-power computers because the actual reflectance signals for measurements through a large range of incidence angles contain a significant amount of data that is affected by a large number of variables. The computational time, for example, can require several minutes such that the substrates are typically evaluated offline instead of being evaluated in-situ within a process tool. Therefore, many conventional scatterometers may not be well-suited for evaluating microstructures on microelectronic workpieces.
Yet another challenge of assessing microstructures using scatterometry is calibrating the scatterometer. One difficulty of calibrating scatterometers is that the return radiation can have both p- and s-polarized components when the input path is off-axis relative to the microfeature (e.g., a grating). This increases the complexity of fitting the output to a model because the p- and s-polarized components must be treated separately. This is also challenging because the p- and s-polarized components change for each off-axis azimuth angle, and thus proper calibration requires measurements and calculations for several different azimuth angles in more sophisticated applications.
Calibrating scatterometers that operate over a large number of azimuth angles is also difficult because it is challenging to measure the p- and s-polarized components. One existing system for measuring p- and s-polarized components is a two-camera system that splits the output beam into separate p- and s-polarized beams which propagate at a non-parallel angle relative to each other. Such systems have one camera to detect the p-polarized component and another camera to detect the s-polarized component. The use of two cameras, however, is undesirable because the additional camera increases the cost and form factor of the scatterometer. This may prevent such two-camera scatterometers from fitting into many integrated tool sets where metrology is desired. Additionally, it is time-consuming to calibrate two cameras because of the additional camera and compensating for the inherent variations in the cameras. Such two-camera systems are also undesirable because the separate images must be registered and integrated with each other to produce a meaningful result. This is a significant, time-consuming computational procedure. Another system for measuring the p- and s-polarized components uses a single camera and a polarizer that alternates between the p- and s-polarized components. This system may have problems because the serial presentation of the p- and s-polarized components to the detector requires more time to obtain the measurements. Moreover, the polarizer is a mechanical device that moves between p- and s-polarizing states, and as such it may lack the precision and accuracy to obtain meaningful measurements. Such mechanical devices may wear out and further denigrate the precision and accuracy of the calibration. Therefore, obtaining images of p- and s-polarized components for calibrating scatterometers or other uses presents a significant challenge in scatterometry.
Still another challenge of scatterometry is noise or inconsequential data in the measurements. In systems that are able to simultaneously obtain measurements through a large range of altitude and azimuth angles, the data in many areas of the resulting image may not be meaningful. Therefore, there is a need to improve the process of operating scatterometers that simultaneously obtain measurements for a large range of altitude and azimuth angles.